Numerous types of intershaft seal can be used to seal between relatively rotating coaxial shafts as found e.g. in gas turbine engines. One type of intershaft seal is shown schematically in FIG. 1 and is used to seal between gas, liquid or air-gas mixtures. The seal is located between inner 37 and outer 38 coaxial shafts and maintains an axial pressure differential between high pressure fluid 31 on a first side of the seal and low pressure fluid 32 on a second side of the seal. The shafts rotate relative to each other. The seal comprises a carbon ring 35 positioned between first 33 and second 34 runners on respectively the first and second sides of the seal. The carbon ring typically has a slight clearance from the outer shaft at cold build conditions. The two runners are separated by a spacer 36, so that the axial gap between the runners is slightly longer than the axial length of the carbon ring. During operation, the carbon ring is pushed by centrifugal forces into more intimate sealing contact with the outer shaft and rotates with that shaft. First 39 and second 40 annular sealing interfaces are created between the runners and the axially spaced first 46 and second 47 end faces of the ring. The sealing interfaces provide only a small axial clearance between each end face and the respective runner.
The series of first runner 33, spacer 36 and second runner 34 is axially restrained relative to the inner shaft 37 by a ring nut 41 (or similar device) which axially clamps these components against a step formation on the inner shaft. Optionally a Belville spring 42 may be used to set the clamping load. The nut arrangement can be located on the opposite (high pressure) side of the seal to that shown in FIG. 1.
During axial movement transients, rubbing contact the respective sealing interface 39, 40 can occur, such rubbing contact producing sufficient axial force to overcome the frictional resistance to sliding axial movement of the carbon ring 35 along the outer shaft 38. Wear of the seal occurs due to rubbing at the end faces 46, 47 of the carbon ring where these contact with the runners 33, 34. Thus an improvement to this technology is the incorporation of lift generating formations, such as V-grooves, in the surfaces of the runners at the sealing interfaces 39, 40. Such formations generate lift as the axial clearance of the sealing reduces, thereby causing the ring 35 to move axially. Ideally this lift can be sufficiently strong such that an air film is produced and no axial rubbing contact occurs under normal operation.
When the carbon ring 35 is midway between runners 33, 34, the seals formed by both sealing interfaces 39, 40 can be inadequate, leading to excessive fluid leakage across the seal. However, reducing the distance between the runners is generally not possible, as provision has to be made for thermal expansion of the ring 35 and manufacturing tolerances. More generally, due to insufficient tolerance control or due to wear of the carbon ring 35, the gaps between the carbon ring and the runners 33, 34 vary from seal to seal and also increase with use. Consequently the seal leakage characteristic varies from component to component and with time.
A development of the intershaft seal, shown schematically in FIG. 2, has the ring split into two halves 43, 44 with a spring element 45 (e.g. a wave spring) positioned between the two halves and urging the halves towards respectively the first runner and the second runner. If this arrangement is combined with lift generating formations which provide sufficient lift to overcome the spring compressive force and hydrostatic pressure acting on the ring halves 43, 44, an air-riding seal is created. Here, the axial clearances of both sealing interface can be kept to a low level (e.g. as low as 2-5 microns), while no wear occurs. Any thermal expansion and build tolerance imperfections can be compensated for by compression of the spring element. Similarly performance degradation due to carbon ring wear throughout the seal life is eliminated.
A disadvantage of the arrangement shown in FIG. 2, however, is that two ring halves 43, 44 with comparatively complicated shapes need to be manufactured and that it can be difficult to position the spring element 45 between the two halves. Significant assembly complications can also be experienced.
A further problem associated with seals of the type shown in FIGS. 1 and 2 is that due to the centrifugal loading and/or differential thermal expansion, the interference between the carbon ring and the outer shaft increases. Consequently as the seal experiences axial motion and as the carbon ring is required to slide axially relative to the outer shaft, a larger frictional force has to be overcome. If this force becomes too large in the case of an air-riding seal arrangement, the air film may not be able to overcome the frictional force and contact can occur. Alternatively in a contacting seal arrangement, increased contact forces can be encountered between the runners and the carbon ring, leading to increased heat generation and wear.